Plasma processing is well known as a surface processing method for executing cleaning or etching upon an object to be processed such as a board on which electronic parts are mounted. In plasma processing, a board to be processed is put into a vacuum chamber forming a processing chamber. Then, plasma discharge is generated in the processing chamber. When ions and electrons generated as a result of the plasma discharge are made to act on a surface of the board, predetermined surface processing is executed. In order to stably execute the plasma processing at high quality, it is necessary that plasma discharge is properly generated according to a condition of plasma discharge which has been previously determined conforming to an object of processing. Therefore, for the object of monitoring a state of the generation of plasma discharge, various means and methods have been conventionally employed.
For example, a method is known which detects an influence exerted upon voltage or current of a high frequency power supply portion by a change in plasma discharge for some factors. Further, a method is known which estimates a state of discharge when a self-bias voltage generated between electrodes by plasma discharge is detected. However, these methods are disadvantageous as follows. In the case where it is necessary to generate plasma discharge under the condition of low output, the detection accuracy is low and it is difficult to accurately detect a state of plasma discharge. Due to the above disadvantages of the conventional methods, a method by which a change in the plasma discharge state can be directly detected has been recently used. Concerning this matter, for example, refer to Patent Document 1. This method detects the existence of abnormal discharge as follows. A discharge detection sensor having a probe electrode for detecting electric potential is attached into a vacuum chamber in which a processing chamber is provided. When a change in the electric potential, which is induced according to a change in plasma discharge in the probe electrode is detected, whether or not abnormal discharge is caused in the processing chamber is detected.    Patent Document 1: Official gazette of JP-A-2003-318115
According to this method, it is possible to highly sensitively detect a change in the state of plasma discharge generated in the processing chamber. Therefore, even when an output of the high frequency power supply portion is low, it is possible in principle to properly monitor whether or not plasma discharge is executed and it is also possible to properly monitor whether or not the plasma discharge is abnormal. However, the above Patent Document does not clearly disclose a specific application example necessary for highly accurately monitor a state of operation including whether or not the plasma discharge is executed, whether the plasma discharge is normal or abnormal and whether or not it is necessary to execute the maintenance work for the vacuum chamber. Therefore, it has been desired to develop a new application technique to be applied to an actual plasma discharge device.